The United States and Soviet Union signed the Strategic Arms Reduction Treaty in 1991; it entered into force in December 1994 and is due to expire in December 2009. The United States and Russia have held several meetings to discuss options for continuing their arms control relationship, but have not agreed on whether to extend START or how to replace it. START counts each deployed ICBM, SLBM, bomber as a single delivery vehicle under the Treaty limit of 1,600 delivery vehicles and attributes an agreed number of warheads to each deployed delivery vehicle. This attribution rule provides the total number of warheads that count under the 6,000 warhead limit in the Treaty. To verify compliance with START, each side monitors the numbers and locations of ballistic missiles, launchers and heavy bombers deployed by the other country. The parties use a wide variety of means to collect information or monitor these forces and activities. Some of these monitoring systems, such as overhead satellites, operate outside the territories of the treaty parties. They also have also been required to exchange copious amounts of data on locations, operations, and technical characteristics of the treaty-limited items. This verification regime has allowed the parties to remain confident in each other's compliance with the Treaty.